The present invention relates to an optical measuring sensor and method for non-invasively measuring the content or composition of one or several chemical components within living tissue, said sensor comprising: a radiation source or radiation sources for emitting measuring radiation with at least two wavelengths to a portion of the body of a patient; detectors or respectively a detector for receiving measuring radiation transmitted through said body portion of a patient and for transforming the same to electrical form; at least one radiation transfer section which is located between either the radiation sources or respectively the detectors and the external surface of said body portion. The invention relates also to the use of such a sensor.
Pulse oximeters are one example for the use of the sensor mentioned above and are capable of measuring the degree of oxygen saturation in a patient's blood non-invasively and continuously. The term non-invasive indicates that the patient is not subjected to subcutaneous penetration by any physical means but the measuring is effected by means of radiation and other than that the procedure occurs externally of the body. This type of monitoring of blood oxygen saturation is quite common today and is one of the monitoring parameters required in many applications. The measurement is optical and based on various absorption characteristics of red and infrared light in blood hemoglobin. The principles of pulse oximetry are disclosed e.g. in Patent publication JP-53-26437. This reference describes a pulse oximeter sensor which comprises a wide-band thermic radiation source and two optical detectors fitted with a bandpass filter for expressing various wavelengths. Other examples for the use of optical sensors mentioned above are a monitor measuring bilirubin concentration in blood as disclosed in publication EP-0 747 002, an instrument analyzing blood glucose as disclosed in publication WO-90/07905, and an apparatus for determining parameters of glucose, protein, albumin, creatinine, carbamide, cholesterol, triglyceride, haemoglobin as disclosed in publication WO-96/37259, etc.
At present, such sensors include two light sources, which are typically light emitting diodes (LED), and a single detector, and the radiation sources are operated alternately in synchronism. In each case, the radiation sources and detectors are included in the sensor itself near a target to be measured and from the sensor extend electrical cables to a control monitor. Usually, the patient experiences no problems when using high-efficiency light sources with a low power consumption. However, the signal-to-noise ratio may sometimes suffer, particularly if for some reason it is desirable to use wavelengths other than those provided by said light sources. In order to achieve as powerful a signal as possible, the light sources can be supplied with a lot of electric power. This may sometimes lead to the situation that, as a result of a higher input of power, the heat generated by light sources affects a target to be measured and causes even burns. Thus, in practice, there is sometimes no choice but to settle for quite a poor signal when sufficient light cannot be produced without coincident heat generation. Neither can the above-described conventional solution be used for example during the course of currently more and more popular magnetic imaging of a patient or while operating some other powerful electromagnetic source. In these conditions, the electrical cable of a sensor would act as an antenna for interferences originating from a control monitor and these interferences would upset the magnetic images. Also other metallic parts at the sensor end disturb a magnetic imager and distort the imaging result. Also in the imaging field, the electric conductor of a sensor may be induced with currents sufficiently strong for a patient to sustain burns or to break down the pulse oximeter equipment. In order to secure more perfect operation in a magnetic imaging environment, the sensor must not include any metallic components at all and, thus, all normal electronics must also be excluded.
Non-invasive optical sensors compatible with the magnetic imaging environment are typically designed with fiberoptics. Hence, the light sources and detector are spaced away from an actual site to be imaged. The light supply is led to and from a site to be measured along optical fibers as described in U.S. Pat. No. 5,103,829. In practice, the fiberoptic cable is a bundle of fibers, comprising a plurality of thin optical fibers and having typically a diameter of 1-3 mm. In order to develop a clinically useful sensor, it is necessary in many cases to bring the bundles of optical fibers to the sensor end in the direction at least roughly parallel to sensor housings. This requires that the light supply be deflected by about 90.degree. inside the sensor housing. The light supply can be deflected by bending the optical fibers to a 90.degree. angle, as disclosed in the publication MEDICAL & BIOLOGICAL ENGINEERING & COMPUTING, Vol. 18/1980 pp. 27-32: Yoshiya, Shimada, Tanaka--"Spectrophotometric monitoring of arterial oxygen saturation in the fingertip." In terms of production, however, this solution is difficult to carry out with sufficiently large bundles of fibers and with a sufficiently small bending radius. The cited glass fibers are easily broken upon bending. Likewise, some of the light supply manages to escape out of the fibers at a sharp curve formed in the fibers.
Other representative fiberoptic designs for a pulse oximeter sensor are described in publications U.S. Pat. No. 5,279,295 and WO-92/21281. In both solutions, the end of a fiber bundle is more or less bent for guiding a light signal to and from a target. In each case, the object of measuring is a patient's finger. In publication U.S. Pat. No. 5,279,295, the monitoring is based on the reflection of light from the finger while publication WO-92/21281 discloses a more conventional solution, wherein the finger is transilluminated. In publication WO-92/21281, the material of a fiberoptic radiation guide is determined to be plastics, i.e. the question is about a plastic fiber, which is probably a little more resistant to bending than glass fiber. However, a drawback in the plastic fiber is that, as pointed out in the publication, its transmittance does not extend very deep into the infrared range, which prevents the use of wavelengths that would be optimal in terms of measuring and also the use of best possible radiation emitting diodes (LED). In these solutions, as well, some radiation may escape out of the sharp fiber curve. The described solutions provide a signal which is poorer than what is achieved if a straight bundle of fibers were orthogonal to the target. Furthermore, the bending of a bundle of fibers is an expensive approach in terms of productivity. Also, the bundle of fibers in the proximity of a target represents a relatively small surface area. A result of this is that the sensor is sensitive to a so-called motion-related artifact. These result either directly from the movements of a target relative to the sensor or from volumetric changes, i.e. absorbency changes, introduced in venous blood by external causes. The harmful effect is further enhanced by the fact that the numerical fiber aperture is relatively small. Normally, a bundle of fibers accepts light rays within the range of .+-.40.degree. or at best .+-.60.degree. with respect to the fiber axis. This is modest with respect to a conventional sensor, wherein the source radiates in principle .+-.90.degree. and also the detector accepts the same amount.
The direction of light supply can also be deflected by means of some external structure, for example a reflective mirror surface. The reflective mirror surface is created e.g. by metallizing a shiny plastic surface for specular reflection or by utilizing a polished prism surface as disclosed in U.S. Pat. No. 5,103,829 for total reflection. However, a metallic or prismatic mirror is not capable of deflecting all radiation supply coming from the fibers. Similarly, a metallic or prismatic mirror is inconvenient in coupling a sufficient amount of tissue-penetrated radiation with a second bundle of fibers and further with the detector. Another downside in a metallic mirror is a metallic layer included therein, which, as a result of evolving eddy currents, may warm up during magnetic imaging and expose the patient to burn hazards. Furthermore, as described above, a metallic layer may disturb magnetic imaging.